1. Field of the Invention
The present invention relates to a gas sensor element and its production method that is built inside an oxygen sensor and so forth which detects the oxygen concentration in exhaust gas that is intimately related to the air-fuel ratio of an air-fuel mixture supplied for combustion in, for example, an internal combustion engine.
2. Description of the Related Art
Gas sensor elements of the oxygen variable concentration electromotive force type that use a ZrO2 solid electrolyte are well known practical examples of gas sensor elements containing a built-in oxygen sensor that are used by installing in the exhaust pipe of an automobile engine.
A gas sensor element capable of detecting oxygen is provided in the leading end of the above oxygen sensor, and the gas sensor element is composed of a bottomed, cylindrical solid electrolyte, a reference gas side electrode on the inside of a reference gas chamber provided inside said solid electrolyte, a measured gas side electrode on the outside of the above solid electrolyte, and a porous protective layer that covers said measured gas side electrode.
A heater may be inserted in the reference gas chamber provided inside the above gas sensor element. The above porous protective layer is composed of multiple layers provided with, for example, a ceramic coating layer or ceramic coating layer and, for example, a γ-Al2O3 layer thereon.
Exhaust gas that passes through an exhaust pipe passes over the above ceramic coating layer and the above γ-Al2O3 layer and reaches the above measured gas side electrode to obtain a sensor output.
However, due to the increasingly severe regulations on emissions in recent years, it has become necessary to control engine combustion more precisely. Consequently, it has become an indispensable condition for gas sensor elements used by containing within an oxygen sensor of the exhaust pipe of an automobile engine to be more stable without changing sensor characteristics despite being exposed to a more severe working environment.
After exhaust gas containing unburnt components has reached the measured gas side electrode, an equilibrium oxygen concentration is obtained due to the occurrence of an oxidation reaction on this electrode, and the output of the gas sensor element is generated according to the difference between this concentration and the oxygen concentration of the atmosphere that has entered the reference gas chamber.
An important characteristic of a gas sensor element output is the λ point at which the output shown in FIG. 4 changes rapidly. Although automobile engine control using an oxygen sensor consists of feedback control by making a judgment of rich or lean with respect to a reference voltage, in order to perform said feedback control precisely, it is extremely important that the above point at which λ changes suddenly (to be referred to as “control λ”) be stabilized. In other words, it is important that control λ always be at a specific position on the curve shown in FIG. 4 regardless of changes in the external atmosphere. Moreover, the responsiveness of the gas sensor element to lean and rich changes is similarly important.
The main factor that causes changes in sensor characteristics such as control λ and responsiveness as mentioned above in the actual usage environment is impaired electrode activity due to the surface of the measured gas side electrode being covered by poisons such as Pb, S and other components of the gasoline used as fuel, as well as gaseous phase silicon (Si) and so forth generated from Si components contained in gaskets and oil and engine seals, that have passed through the porous protective layer of the gas sensor element and reached the surface of the measured gas side electrode.
Furthermore, this phenomenon is also referred to as poisoning deterioration of a gas sensor element.
Japanese Examined Patent Publication No. 8-10210 proposes a method for preventing deterioration of sensor characteristics caused by Si poisoning by containing an Si reactive component comprised of one or more types of IIIa subgroup elements of the periodic table and their compounds (excluding oxides) in a porous protective layer base material.
The above art offers the effect of preventing adherence of Si only by chemically reacting Si components.
However, in cases of more severe endurance conditions, in other words, when exposed for long periods of time to Si or other poisons at high concentrations, the prior art is unable to obtain adequate effects for preventing Si poisoning, thus resulting in the problem of conspicuous deterioration of the gas sensor element.
Since compounds containing IIIa subgroup elements and silicates of IIIa subgroup elements typically have a comparatively high melting point, they cannot be said to be adequately effective in trapping Si and other poisons in a porous protective layer under conditions of low temperature, high Si concentration and a long endurance period.
Japanese Unexamined International Patent Publication No. 6-502014 is another example of the prior art.
This publication describes the generation of an advantageous gettering action in opposition to Si, Pb and other poisons by containing a mixed oxide comprised of an alkaline metal oxide and heat-stable metal oxide containing a trivalent element in a porous protective layer of a gas sensor element.
However, in this example of the prior art, since the gettering material (material that primarily traps poisons) is a mixed oxide of an alkaline metal oxide and trivalent or tetravalent oxide, when used for a long period of time, free alkaline oxide components become unstable due to a usage environment in which the gas sensor element is exposed to rapid temperature changes and atmospheric changes, resulting in increased susceptibility to changes in other stable compounds such as alkaline carbonates and hydroxides.
In the case a free alkaline oxide component has changed, the volume expands which leads to clogging of the porous protective layer and separation of the porous protective layer, thereby resulting in the risk of a decrease in the poison trapping action and a decrease in the responsiveness of the gas sensor element.
In this manner, in the case of the poison trapping technology of the prior art, it was difficult to maintain a stable gas sensor element output over a long period of time under conditions such as a low atmospheric temperature or high poison concentration and so forth.